The present invention relates to a white light emitting element and a white light source comprising the white light emitting element which are applicable to, e.g., a white light emitting diode.
In a GaN-based III-V compound semiconductor (hereinafter referred to as “GaN-based semiconductor”) represented by AlxGayIn(1−x−y)N(0≦x≦1, 0≦y≦1, 0≦1−x−y≦1), the forbidden band width of GaN at a room temperature is 3.4 eV, which indicates that the GaN-based semiconductor is a material capable of implementing a high-output light emitting element which emits blue or green light. The GaN-based semiconductor has already been commercialized as a blue/green light emitting diode in various displays including a large-scale display and a traffic signal. Further, the use of the GaN-based semiconductor for the light emitting element of a white light source has also been examined.
If such a white light source can be realized, semiconductor illumination as a replacement for currently prevailing fluorescent and incandescent lamps can be implemented. Accordingly, vigorous research and development has been promoted with the aim of enhancing brightness and improving a light emitting efficiency. For example, a white light source in which a blue light emitting diode (hereinafter referred to as “GaN-based blue light emitting diode”) using a nitride semiconductor as a light emitting element is used for an excitation light source and a YAG fluorescent material is used for a white light emitting element is disclosed in, e.g., S, Nakamura et al., “The Blue Laser Diode” Springer-Verlag Berlin Heidelberg New York. In the white light source, the YAG fluorescent material is irradiated with blue light (light with a peak wavelength of about 470 nm) from the blue light emitting diode so that a portion of the blue light is absorbed by the YAG fluorescent material. As a result, the YAG fluorescent material emits yellow light, while the remaining portion of the blue light passes through the YAG fluorescent material without being absorbed thereby. Thus, the irradiation of the YAG fluorescent material with blue light causes the emission of white light composed of blue light and yellow light, as is disclosed in the foregoing document. The white light source has already been commercialized and used for various displays and the like.
To use the white light source for illumination, however, the improvement of the manner in which the white light source used for illumination appears, i.e., a color rendering property is important in addition to the enhancement of brightness and the improvement of the light emitting efficiency. The emission spectrum of the white light source is composed only of yellow light and blue light and therefore the ratio of the red component to the other color components is low. As a result the light emitted from the white light source has a poor color rendering property as white light.